The present invention relates to an electrostatic actuator which generates pressure by displacing a vibrating plate with electrostatic force, and to a manufacturing method therefor, and also relates to a liquid spraying or discharging device, such as an ink-jet head, for discharging droplets, using the same. More particularly, the present invention relates to an electrostatic liquid spraying or discharging device such as an ink-jet head printer, for example, capable of continually performing proper droplet discharging action regardless of external atmospheric pressure fluctuations.
The background art will now be described with an ink-jet printer as an example of an electrostatic liquid discharging device. An ink-jet printer having an electrostatic ink-jet head is disclosed in Japanese Unexamined Patent Application Publication No. 6-55732, for example. With this type of ink-jet head, ink within a pressure chamber is discharged from an ink nozzle by vibrating a vibrating plate forming a portion of the pressure chamber wherein ink liquid is stored, by electrostatic force. Accordingly, changes in the external atmospheric pressure alter the discharge properties of the ink droplets, which may cause a problem in that the desired ink droplets may not be discharged.
That is, with an electrostatic ink-jet head, a vibrating plate defining a portion of the pressure chamber faces an electrode plate across a narrow gap, and the vibrating plate is vibrated by electrostatic force by applying a driving voltage therebetween. The gap between the vibrating plate and the electrode plate is extremely narrow, around 1 to 2 microns, so the space between the vibrating plate and the electrode plate is sealed to form a sealed chamber, so that the vibration of the vibrating plate is not inhibited by intrusion of dust and the like therebetween.
In the event that the external atmospheric pressure fluctuates, the vibrating plate, partitioning off the pressure chamber and the sealed chamber, is displaced in a direction such that the pressure within the sealed chamber matches that of the external atmospheric pressure. Consequently, even when voltage has not been applied, the vibrating plate is in a state of already having been displaced. Thus, in the event that the external atmospheric pressure fluctuates, the vibrating properties of the vibrating plate change even when the same driving voltage is applied, and the discharging properties of the ink droplets (the amount of ink droplets per discharge and the discharging speed of the ink droplets) change.
Now, ink-jet printers having a bubble-jet ink-jet head disclosed in Japanese Unexamined Patent Application Publication No. 4-284255 are known. This Unexamined Patent Application Publication discloses a method for performing continuously stable ink droplet discharging action regardless of fluctuations in the external atmospheric pressure, by detecting the ambient atmospheric pressure and changing the voltage waveform applied to the electro-thermal converting member, that is to say, the driving voltage waveform of the ink-jet head, according to the external atmospheric pressure.
This method is effective for ink-jet heads according to the bubble-jet method wherein ink liquid within the pressure chamber is heated and caused to bubble, but is seldom advantageous when applied to electrostatic ink-jet heads. Particularly, in environments such as high elevations where the atmospheric pressure differs markedly from normal, merely adjusting the driving voltage waveform for driving the vibrating plate may not be sufficient to facilitate appropriate droplet discharge.
Now, electrostatic actuators may be applied to other devices besides the ink-jet printer given here as an example, such as fuel injection devices for internal combustion engines, atomizers for spraying liquids such as perfume, and micro-pumps, but with these devices as well, it is thought that the droplet discharge properties fluctuate according to fluctuation of external atmospheric pressure.
It is an object of the present invention to provide an electrostatic actuator capable of reliably generating desired pressure unaffected by fluctuations of the external atmospheric pressure, and an electrostatic liquid spraying device capable of discharging droplets in an appropriate manner.
In order to achieve the above objects, the electrostatic actuator according to the present invention comprises a vibrating plate, an electrode plate facing the vibrating plate, and a sealed chamber formed between the electrode plate and the vibrating plate; wherein the vibrating plate is displaced through electrostatic force by applying voltage between the vibrating plate and the electrode plate; the electrostatic actuator having a pressure compensator for decreasing the pressure difference between the internal pressure in the sealed chamber and the external pressure.
The pressure compensator employed may comprise a pressure compensating chamber communicating with the sealed chamber, and may be capable of increasing/decreasing its volume according to external atmospheric pressure.
In this case, the entire pressure compensating chamber might be formed of an expanding/contracting material, or a portion of the pressure compensating chamber may be defined by a displacement plate displaceable in the direction perpendicular to its plane (horizontal plane) according to external atmospheric pressure.
Now, the displacement plate is only slightly separated from the facing wall of the pressure compensating chamber, so in the event that the external atmospheric pressure is high, the displacement plate may be displaced and come into contact with the opposing inner wall, thereby inhibiting pressure compensating functions. Also, in the event that the displacement plate is towards the facing wall the compliance thereof decreases, which may also result in the inhibition of pressure compensating functions. Accordingly, the displacement plate is preferably curved in a form so as to protrude in a direction away from the facing inner wall of the pressure compensating chamber.
Next, an arrangement may be used wherein a displacement plate displaceable in the outwards direction of the plane is positioned as a portion of the horizontal pressure compensating chamber, wherein the displacement plate and electrode plate are positioned facing one another, so that the displacement plate is displaced according to changes in external atmospheric pressure, by electrostatic force.
Alternately, the pressure compensator may comprise a heat-generating member capable of at least heating gas sealed in the sealed chamber, instead of the pressure compensating chamber. Heating the sealed gas with the heat-generating member raises the internal pressure thereof, so the pressure difference with the external atmospheric pressure can be relieved.
A preferable material used for configuring the electrostatic actuator is a semiconductor substrate which can be worked with precision. Accordingly, for example, doping a semiconductor substrate with boron, etching the semiconductor substrate, and using the boron doped layer as a displacement plate, allows a displacement plate with desirable properties (compliance) to be obtained. Also, in order to form the electrostatic actuator in a compact size, the vibrating plate and displacement plate are preferably sectioned and formed using a common semiconductor substrate.
Also, the electrostatic liquid spraying or discharging device according to the present invention uses the pressure of the above electrostatic actuator having the pressure compensating functions, as the pressure generating source for discharging droplets. That is, the electrostatic liquid spraying or discharging device comprises a nozzle for discharging droplets, and a pressure chamber communicating with the nozzle and also holding liquid, wherein a vibrating plate provided in a portion of the pressure chamber is vibrated by the above-described electrostatic actuator, thereby providing the liquid in the pressure chamber with pressure fluctuation for discharging droplets.
Common ink-jet heads serving as liquid discharging devices are provided with multiple ink nozzles, with pressure chambers provided corresponding to each ink nozzle, and with a common ink chamber (common liquid chamber) for supplying ink liquid to the pressure chambers provided.
Now, a diaphragm displaceable in the outwards direction of the horizontal plane is in some cases formed at the common ink chamber, so that pressure fluctuations in each of the communicating pressure chambers are not transmitted to the side of the neighboring pressure chamber. In the case of applying the present invention to such an ink-jet head, the diaphragm may also serve as the displacement plate. Also, in order to form the device or a portion thereof (the ink-jet head) in a compact size, the pressure chamber, the common ink chamber, and the pressure compensating chamber are preferably sectioned and formed using a common semiconductor substrate.
In particular, with a liquid spraying or discharging device comprising an electrostatic actuator of a configuration wherein the displacement plate of the pressure compensating chamber is displaced by electrostatic force, the configuration may have an external atmospheric pressure detector for detecting external atmospheric pressure, and a controller for driving the displacement plate according to the detected external atmospheric pressure.
Also, with a liquid spraying or discharging device having an electrostatic actuator comprising the heat-generating member, the configuration may have an external atmospheric pressure detector for detecting external atmospheric pressure, and a controller for driving the heat-generating member according to the detected external atmospheric pressure.
Now, the external atmospheric pressure detector may be of a configuration comprising an electrostatic capacity detector for detecting the electrostatic capacity between the vibrating plate and the electrode plate, and thereby estimating the external atmospheric pressure based on the detected electrostatic capacity.
The method for manufacturing the electrostatic actuator according to the present invention is a method for manufacturing an electrostatic actuator comprising a vibrating plate, an electrode plate facing the vibrating plate, and a vibrating chamber formed between the electrode plate and the vibrating plate, wherein the vibrating plate is displaced by electrostatic force, by applying voltage across the vibrating plate and the electrode plate, the method comprising: a process for forming a pressure compensating chamber communicating with the vibrating chamber; a process for forming a displacement plate at a portion of the pressure compensating chamber, displaceable according to external atmospheric pressure, into a form curved so as to protrude in a direction away from the facing inner wall of the pressure compensating chamber; and a process for shutting off and sealing the pressure compensating chamber from the external atmosphere, along with the vibrating chamber. Furthermore, the air pressure for sealing the pressure compensating chamber may be adjusted. Accordingly, the initial curvature of the displacement plate is adjusted, and a displacement plate with desired compliance properties can be obtained.